Anionic polymerization is widely used for the synthesis of block copolymers. Anionic systems are generally attractive for a variety of reasons. Products have a predictable molecular weight equal to the grams of monomer divided by the moles of initiator, and a narrow molecular weight distribution with high purity block compositions. Facile and efficient paths are available for coupling of living segments into linear, branched, or star-shaped species or, a second anionically polymerized block might be polymerized to afford an ABC three block copolymer. Also efficient end-of chain functional capping is possible with reagents such as ##STR1## etc. However only activated olefins such as styrene monomer, butadiene, isoprene, methylmethacrylate, etc., are susceptible to this mode of polymerization. It is well known in the art that alkyl lithium reagents will not readily catalyze the polymerization of relatively inexpensive ethylene propylene, or butylene monomers.
Ziegler-Natta polymerization is an important mechanism for the polymerization of olefins. Ziegler-Natta catalytic systems are defined as a combination of metal alkyls of groups I to III with transition metal salts of groups IV to VIII. This method is effective in polymerzing a wide range of olefin monomers including ethylene propylene, 1-butene, and mixtures thereof
It is also well known that in the catalytic polymerization of alpha olefinic hydrocarbons such as propylene or butene, it is possible to produce polymers having widely different properties and physical characteristics depending, to a large extent, upon the catalyst system and the process conditions. Much of the work in this field has been directed to the development of catalysts and catalytic processes that are capable of forming highly crystalline polymers, of at least 70% crystallinity, since it has been shown that these highly crystalline polymers have greatly improved properties over the completely or predominantly amorphous polymers. It is apparent therefore, that processes that will form highly crystalline polymers are of considerable importance in the art. Ziegler-Natta catalysis has been found to produce highly isotactic homopolymers which have an increased crystallinity and a higher melt temperature. Also block copolymers containing isotactic or crystalline blocks will have higher service temperatures than their atactic amorphous analogs.
Ziegler-Natta catalysts have also been employed in the copolymerization of ethylene, propylene, and/or 1-butene. Inexpensive elastomers are produced in this way. Block copolymers having inexpensive rubber segments would enjoy cost advantage over analogous materials based upon butadiene or isoprene.
Since the inception of Ziegler-Natta catalysis numerous researchers have attempted to use this polymerization technique to synthesize either sequential olefin-olefin or olefin-vinyl block copolymers in cases in which the polar monomer is compatible with such catalysts. In spite of considerable work in the area it has not been possible to synthesize such block copolymers. Sequential copolymers of the olefin type that can be obtained through the Ziegler-Natta catalysis are swamped with large amounts of corresponding homopolymers. This difficulty stems from the fact that in Ziegler-Natta catalysis the average life of nascent chains is very short, which is primarily due to transfer reactions. The larger the number of blocks desired the more difficult it is to obtain a selected polyolefin block copolymer. The precise segmented structure which can be obtained with the long-lived anionic systems is simply not possible with the Ziegler-Natta catalysts.
New approaches have been developed in recent years for the synthesis of block copolymers involving novel combinations of monomers resulting in novel properties. These have required various transformation reactions to enable the mode of polymerization employed for the first polymer to be switched to a different mode most suitable for polymerizing the second monomer. Such processes may require an intermediate stage where the initial homopolymer is isolated before introducing it into an environment appropriate to the polymerization of the second monomer. Examples have been published of the following transformation reactions: Anion to cation, anion to free radical, cation to anion, anion to Ziegler-Natta catalysis and recently Ziegler-Natta catalysis to free radical catalysis.
The Ziegler-Natta to free radical scheme consists of polymerizing olefin in the presence of Ziegler-Natta catalysts and a third organometallic compound usually diethylzinc or diethyl cadmium then activating these terminal metal-carbon bonds with a cocatalyst such as oxygen, peroxide, etc. which gives rise to free radicals and initiates polymerization of the vinyl monomers. This process, however, is complicated by the formation of graft and homopolymers.
Agouri et al in U.S. Pat. Nos. 3,851,015 and 3,949,016 used this technique for preparing bi-sequenced copolymers of the polyolefin-polyvinyl type which contained both a crystalline and an amorphous sequence.
In a first phase Agouri polymerized an olefin using a conventional Ziegler-Natta system in the presence of diethylzinc which is a very effective transfer agent for this type of polymerization. The second sequencing phase consisted of using an oxidizing agent to activate the zine-carbon bond located on the polymer so as to initiate the free radical polymerization.
Ziegler-Natta catalysis is capable of producing highly isotactic therefore highly crystalline polymers and in addition can polymerize a wide range of monomers including ethylene and propylene. Zielger-Natta catalysis however results in polymers with very short life times making sequential polymerization difficult or impossible. A method was sought therefore that could render the Ziegler-Natta polymer products living. The process of the present invention requires a transfer from a Ziegler-Natta catalyst to an anionic polymerization initiator. This Ziegler-Natta to anionic route attaches the second block via a living polymerization reaction. This living polymer scheme allows further reaction of the di-block polymer such as end capping, coupling, polymerization of a third or further block or grafting. In addition, the transfer reactions are accomplished by straight forward irreversible chemistry. Block copolymers yield is very high due to the absence of side reactions and/or unfavorable equilibria. This route allows, for the first time, the direct synthesis of elastomers of isoprene which may be hydrogenated to a copolymer of ethylene-propylene or butadiene which may be hydrogenated to a copolymer of ethylene-butylene with crystalline, Ziegler-Natta (Z-N) endblocks of propylene, 1-butene or ethylene.